1. Field of the Invention
The present invention relates to micro-fabrication techniques for making electron beam confining structures. In particular, the present invention relates to slow wave structures (SWSs) for microwave amplifiers and oscillators and methods for micro-fabricating such SWSs.
2. State of the Art
A traveling wave tube (TWT) is generally used to provide microwave generation and microwave amplification. A conventional TWT typically includes a slow wave circuit or structure defined by a generally hollow vacuum-tight barrel with optional additional microwave circuitry disposed inside the barrel. An electron source and suitable steering magnets or electric fields are arranged around the slow wave circuit to pass an electron beam through the generally hollow beam tunnel. In a conventional TWT, an electron beam interacts with a propagating electromagnetic wave to amplify the energy of the electromagnetic wave. This interaction may be achieved by propagating the electromagnetic wave through a structure which slows the axial propagation of the electromagnetic wave and brings it into synchronism with the velocity of the electron beam. The kinetic energy in the electron beam is coupled into the electromagnetic wave, thereby amplifying the electromagnetic wave. Such a structure may be referred to as a “slow wave structure”. Conventional slow wave structures may take the form of, e.g., a circular, square or hexagonal cross sectioned generally hollow structure surrounding the electron beam.
Various methods for constructing helixes for use in TWTs are known in the art. Common fabrication techniques include winding or machining. For example a thin wire or tape of conductive material may be wound around a mandrel and processed to properly shape the helix to a circular configuration. Drawbacks associated with the winding technique include placing stress on the wire or tape, which may limit stability of the helix under operating conditions. Additionally, when heated during annealing or operation, Wound helixes lack dimensional stability because of, e.g., physical distortion.
Another conventional approach to forming cylindrical helixes suitable for TWTs involves cutting a cylindrical tube into a desired helix pattern using electrical discharge machining. But, such helical structures formed according to this technique tend to be brittle and subject to cracking. Additionally, both conventional winding and machining techniques become impractical when used for high frequency applications because of the need for smaller dimensions associated with higher frequencies.
U.S. Pat. No. 5,112,438 to Bowers discloses a photolithographic method of forming helixes for TWTs. Bowers discloses the use of a mandrel on a lathe as a form for micro-fabricating a SWS. Using conventional planar processing techniques Bowers builds the SWS on the mandrel and then separates the SWS from the mandrel. However, the Bowers approach appears to require sophisticated linear and rotation control during processing.
Yet another prior art approach to forming cylindrical helixes suitable for TWTs is disclosed in U.S. Pat. No. 6,584,675 to Rajan et al. Rajan et al. discloses a method for fabrication of three dimensional TWT circuit elements using laser lithography. According to the method of Rajan et al., a small hollow preform (square or cylindrical tube) is coated with a layer of photoresist material, patterned, stripped and etched and optionally polished. However, Rajan et al. requires an ultraviolet (UV) laser and, like Bowers, a sophisticated linear and rotation controller for processing the preform. Additionally, the method of Rajan et al. requires significant exposure time (1-2 hours) which limits its use for mass production. A similar technique is disclosed in U.S. Patent Application Publication No. U.S. 2003/0151366 to Dayton, Jr. The Dayton, Jr. device also requires expensive laser micromachining for fabrication.
Lithographic techniques are regularly used in the electronics industry to achieve small features required for high frequency electronics. However, these techniques are generally applied to planar wafer substrates or silicon or other semiconductor materials. As such, lithographic techniques have not been readily adapted to produce the types of finely detailed three-dimensional structures called for in TWTs and other high frequency devices designed for high frequency operation.
Thus, there exists a need in the art for new micro-fabrication techniques for SWSs for TWTs and other microwave amplifier and oscillator devices that utilize lithographic techniques for mass production without the drawbacks associated with conventional methods of manufacturing SWSs.